Thanks. Do you know how common it is to use it like that? I've never seen it used before so I would guess it's used rarely, but I fell as though most situations when your talking about a cos or a lámh would be ambiguous and call for it.

It’s a well-aged interjection, at least on the western side of the Atlantic, if not on the eastern side — though its use as an interjection would not be literally translated into Irish. (The New English-Irish Dictionary offers Mhuise! or Mo thóin! as translations — the latter is noted as “very informal”.)